This invention relates to a variable capacity type vane pump for use in power steering unit or the like.
Automotive vehicles employ variable capacity type vane pumps having its discharge amount variable in accordance with the speed of rotation of the pump to control power steering control. For example, Japanese Utility Model Kokai No. 59-159793 discloses a conventional variable capacity type vane pump. The conventional vane pump, shown in FIG. 7, includes a pump housing A which houses a drive shaft B, a cam ring C and a rotor D fixed on the drive shaft B for rotation in unison therewith within the cam ring C. The drive shaft B is drivingly connected to the engine to rotate in a counter-clockwise direction, as viewed in FIG. 7, indicated by the arrow. The rotor D has a number of vanes E provided for radial movement in respective radial slot F equally spaced circumferentially of the rotor D. With rotation of the drive shaft B, the rotor D rotates with the vanes held in sliding contact with the inner cam surface of the cam ring C to introduce oil into the vane pump through an inlet port G and discharge pressurised oil from the vane pump through an outlet port H. The pressurized oil is supplied to a power steering control valve through an output conduit I having a restriction orifice J.
A control mechanism K is provided to control the concentric amount e of the center 02 of the cam ring C with respect to the center 01 of the rotor D so as to vary the displacement of the vane pump. This control is made based upon an oil pressure differential across the restriction orifice J. For this purpose, the control mechanism K receives an oil pressure P1 introduced thereinto through a conduit L connected to the output conduit I at a position upstream of the restriction orifice J and an oil pressure P2 introduced thereinto through a conduit M connected to the output conduit I tat a position downstream of the restriction orifice J. As the speed of rotation of the drive shaft D increases, the amount of oil discharged through the outlet port H increases to increase the pressure differential (P1-P2) across the restriction orifice J. When the pressure differential across the restriction orifice J reaches a predetermined value, the control mechanism K rotates the cam ring C from its maximum eccentric position (FIG. 7) in a direction decreasing the eccentric amount e so as to decrease the amount of oil discharged through the outlet port H.
With the conventional vane pump, however, the fluid pressure P1 remains much greater than the fluid pressure P2. This causes a vane pump inner pressure increase which, in turn, causes power loss and oil temperature increase. This is stemmed from the fact that the restriction orifice J is provided in the output conduit I.